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Glacial Till and Glacial Flour

The sediments produced through glacial grinding are very distinctive. Glacial till contains sediments of every size, from tiny particles smaller than a grain of sand to large boulders, all jumbled together. Glacial flour is that smallest size of sediment (much smaller than sand) and is responsible for the milky, colored water in the rivers, streams, and lakes that are fed by glaciers.

A road cut through a moraine in Yellowstone National Park exposes the glacial till inside. This till includes large rocks that can be picked out within the photo as well as the small and tiny grains that surround them.

NPS Photo/John Good

Glacial till is the sediment deposited by a glacier. It blankets glacier forefields, can be mounded to form moraines and other glacier landforms, and is ubiquitous in glacial environments. It has a very distinctive composition that arises from the fact that glaciers not only grind rocks, breaking them into small, fine pieces but also they also pluck off large chunks of rock. This means that glaciers transport everything from large boulders to tiny grains smaller than sand.

These rocks and sediments are all mixed together in a jumble after they are deposited. In contrast, rocks and sediments deposited by rivers settle out as the water speed slows, so big boulders are often dropped before small grains of sand. Rather than jumbling sediments of every size, rivers sort them out in a way that viscous glaciers cannot.

Glacial Flour

The waters coming off of the Guyot Glacier at its terminus in Icy Bay (Wrangell-St. Elias National Park, Alaska) are cloudy with suspended glacial flour. The difference between the highly-concentrated sediment in the bay near the glacier and that of the more dilute water on the right is striking.

NPS Photo/Jacob W. Frank

Glacier flour describes the component of glacier sediment that is much finer than sand. This material has a similar consistency to flour, which is the reason for its name. Because this sediment is so fine, it is easily transported by and suspended in water. It is responsible for the cloudy or milky appearance of the streams, rivers, and lakes that are fed by glaciers. Glacier lakes can have a wide range of beautiful colors that arise as sunlight scatters when it hits sediment particles in the water.